Hitherto, photopolymerizable compositions and negative-working lithographic printing plate precursors using the same have been widely known, and various photosensitive layers are also known, In order to conduct plate making of such a lithographic printing plate precursor, a method wherein a transparent negative film original (lith film) is put on the lithographic printing plate precursor and the lithographic printing plate precursor is imagewise exposed using an ultraviolet ray is ordinarily used and thus, the method involves a large amount of time and effort to conduct plate making.
On the other hand, digitized techniques of electronically processing, accumulating and outputting image data using a computer have prevailed in recent years, and various image output systems corresponding to these digitized techniques have been put into practical use. Also, photopolymers having high sensitivity to radiation used for the exposure has been requested. Specifically, photosensitive materials suitable for plate making using, for example, a visible light laser or a near infrared ray laser are desired and a radiation polymerization system is a promising photosensitive material because of the highest sensitivity. As an exposure apparatus for such a system, for example, 488 nm and 514.5 nm rays of an Ar laser, 390 to 410 nm ray of a GaN semiconductor laser, 532 nm ray of a FD-YAG laser or 830 nm ray of a high power semiconductor laser has been practically used.
By using a high-sensitive polymerizable photosensitive layer as a photosensitive layer, a direct plate making is made possible. For instance, the direct plate making can be performed by using photosensitive compositions described in Patent Documents 1 to 3 shown below.
Also, it has been ordinarily conducted to provide a protective layer comprising a water-soluble resin on the photosensitive layer in order to protect the photosensitive layer from stain and dust and to prevent a monomer component conventionally used in the photosensitive layer from being deactivated due to reaction with oxygen in air. However, in the case of using the laser exposure apparatus described above, since plate making is conducted using the laser exposure apparatus having an automatic transport mechanism, problems occur in that an inter leaf, which is interleaved between the lithographic printing plate precursors for the purpose of protecting the surface of the lithographic printing plate precursor, adheres on the surface of protective layer of the lithographic printing plate precursor or friction coefficient of the surface of protective layer increases even though the adhesion doesn't occur and thus the transport and removal of the inter leaf becomes impossible or the transport of the lithographic printing plate precursor becomes impossible in the step of transport of the lithographic printing plate precursor and the inter leaf and in the step of removal of the inter leaf, when environmental humidity is high. Alternatively, even though the transport of the lithographic printing plate precursor can be conducted, the surface of the protective layer is scratched to cause failure in the image formed in some cases.
Patent Document 1: JP-B-61-9621 (the term “JP-B” as used herein means an “examined Japanese patent publication”)
Patent Document 2: JP-A-63-178105 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 3: JP-A-2-244050